5.0Torx - With only four events under your belt and no autocross experience that I know of, you're going to be happy because (as near as I can tell) you haven't done enough driving at that hard of a level. Hard street driving, or what you may think was hard street driving, doesn't count for much here. No flame intended.
For all of the cars I ever bought aftermarket wheels for, the wheels were always picked in a width out toward max recommended for the intended tire size, and that's a history that reaches back 40+ years. Mainly for the improvements in turn-in and steering precision. Tire sizes themselves were picked with car weight in mind first.
Rather recently I picked up an Aim Solo datalogger, so I can put some numbers to go with evaluations of subjective feel. On my '08 GT, with (at the time) Koni yellows and firmer sta-bars and a moderately aggressive alignment, I tested my street wheel/tire set (MPSS in 265/40-18 on 18x9.5 wheels) and compared it to my track wheel/tire set (MPSS 285/35-18 on 18x11's). Same sizes all around, no width staggering for me. On one of my unofficial "test loops", the track setup was easier to drive at and more composed at 0.91 lateral g than the street setup was at 0.84 lateral g. I can explain only part of the difference as coming from the roughly quarter inch difference in sidewall height.
The track setup has datalogged in excess of 1.3g on a pretty flat track (NJMP). Even accounting for body roll introducing datalogging error, that's still about 1.2g or so, on 300 treadwear tires. Granted, this was on tires that were fully up to track temperature, but it was repeatable as long as I did my part.
I'll stick my neck out a bit to suggest that fitting tires to max-recommended width wheels probably demands a smoother driving style. Otherwise, any extra overshoot in roll could pick up more of the inside tread region of the outboard front tire.
I really doubt that I'd ever choose to run 275/40's on anything narrower than 10". Hell, I'd have 10's for the 265/40's if I was starting from scratch (the 9.5's were bought with 255/45's specifically in mind, just like what the 2008 GT500 wore up front). Apparently Ford was being unusually generous back around 2008; in addition to the GT500 fitment the "regular" GT got 235/50-18's on 8.5" wides.
Norm, thanks for the reply. Good science is good observation, so its good to hear from someone that has put the time and effort into getting good data. I don't take any offense to your comment on my skill level. I know full well that a pro driver in a half-broken Miata with 2 cylinders unplugged could smoke my ass lol. I'm learning and getting better every time I go out and I'm having a blast doing it, that's enough for me at this time.
I mean no disrespect either when I say I am curious as to where you comment on "hard street driving" came from, though. I never bought my 99 GT for that purpose. The previous owner of my car ran it in NASA TTC and I scooped it up when he moved out of state. My dd is my '94 5.0 GT which I'm done some little stuff to, nothing as major as what's done to the '99 (MM TA+PHB, Cobra brakes, other stuff), and I actually did do my very first motorsports thing ever, my one and only autox event, in my '94. After that I bought the '99 and have been doing track days ever since. I honestly dont really have any interest in doing any more autox. I know there's alot of overlap and also skills unique to autox that you might not get on a track, but, the dollar-to-drive-time ratio of track days absolutely creams autox, and you also get way more opportunity to learn or try new things from lap to lap, rather than having to wait another hour. Anyways, not quite sure where the street driving comment came from but I only posted this thread for road racing / track days driving.
After reading this thread I think my next tires will probably be 255/40s on my 17x9s because I dont care to drop $1000+ on all new bigger wheels and tires.
This past weekend I ran my car at Barber Motorsports Park, so, I'm up to 5 TDs now!